ARM: 8318/1: treat CPU feature register fields as signed quantities

The various CPU feature registers consist of 4-bit blocks that
represent signed quantities, whose positive values represent
incremental features, and whose negative values are reserved.

To improve forward compatibility, update the feature detection
code to take possible future higher values into account, but
ignore negative values.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 16 insertions, 0 deletions

diff --git a/arch/arm/include/asm/cputype.h b/arch/arm/include/asm/cputype.h
index 819777d0e91f..85e374f873ac 100644
--- a/arch/arm/include/asm/cputype.h
+++ b/arch/arm/include/asm/cputype.h
@@ -253,4 +253,20 @@ static inline int cpu_is_pj4(void)
 #else
 #define cpu_is_pj4()    0
 #endif
+
+static inline int __attribute_const__ cpuid_feature_extract_field(u32 features,
+                                                                  int field)
+{
+        int feature = (features >> field) & 15;
+
+        /* feature registers are signed values */
+        if (feature > 8)
+                feature -= 16;
+
+        return feature;
+}
+
+#define cpuid_feature_extract(reg, field) \
+        cpuid_feature_extract_field(read_cpuid_ext(reg), field)
+
 #endif